1. Technical Field
The present invention relates to the field of top spin-valve sensors.
2. Background Art
Spin valve sensors exploit changes in electrical resistance which occurs as a result of manipulating the relative orientation of the magnetization of ferromagnetic layers within a spin valve sensor. In conventional spin valve sensors, one ferromagnetic layer has its magnetization pinned while another, which has its magnetization set perpendicular to the pinned layer, is free to change its magnetic orientation in response to magnetized bits on
R↓↑ is the resistance across the sensor when the magnetization of the layers are not aligned
R↓↓ is the resistance across the sensor when the magnetization of the layers are aligned.
Spin valve layout and materials selection for the spin valve is critical to optimizing the GMR effect and sensor performance.
As data bits are made smaller to increase data density, the magnetic field generated by the smaller bits becomes weaker. Thus, there is less magnetic field to rotate the free layer. As a result, the free layer must be made thinner so it can be saturated by the weaker magnetic fields as discussed in U.S. patent application Ser. No. 09/356-617, by Anderson and Haui, entitled ULTRA THIN FREE LAYER SPIN-VALVE DEVICE WITH ENHANCEMENT LAYER, filed on Jul. 19, 1999, herein incorporated by reference in its entirety.
As free layer thickness is reduced, it becomes more important to establish good texture throughout the entire free layer and spin valve. Conventional spin valves are formed on a tantalum seed layer to provide desired crystalline texture. Although a tantalum seed layer does provide a suitable texture for conventional free layers, as free layer thickness is reduced, the tantalum seed layer does not provide optimum texture for the free layer. This is because tantalum does not immediately establish good texture in the free layer material but rather facilitates the free layer to gradually establish good texture. Thus, as free layer thickness is reduced, the tantalum seed layer an adjacent recording media. The magnetized bits on the recoding media, therefore, change the relative magnetization between the pinned layer and the free layer. A sensing current through the spin valve is used to detect changes in the resistance of the spin valve that results from changes in the relative magnetization of the pinned and free layers.
Examples of this spin valve sensors may be found in U.S. Pat. No. 5,206,590, by Dieny et al., entitled MAGNETORESISTIVE SENSOR BASED ON THE SPIN VALVE EFFECT, issued on Apr. 27, 1993; in U.S. Pat. No. 5,701,223, by Fontanta et al., entitled SPIN VALVE MAGNETORESISTIVE SENSOR WITH ANTIPARALLEL PINNED LAYER AND IMPROVED EXCHANGE BIAS LAYER, AND MAGNETIC RECORDING SYSTEM USING THE SENSOR, issued on Dec. 23, 1997; and in U.S. patent application Ser. No. 09/135,939, by Huai and Lederman, entitled SYNTHETIC ANTIFERROMAGNETIC STRUCTURE FOR USE IN A SPIN-VALVE DEVICE AND METHOD OF FABRICATION, issued on Jan. 16, 2000 as U.S. Pat. No. 6,175,476, all herein incorporated by reference in their entireties.
As discussed above, the magnetic moment on the magnetic media changes the resistance across the spin valve which can be detected by passing a current through the spin valve. The giant magnetoresistance, one measure of the performance of a spin valve, is given by:
GMR=(R↓↑xe2x88x92R↑↑)/R↑↑
where,
GMR is the giant magnetoresistance ratio is no longer able to impart sufficiently good texture to provide optimum spin valve performance.
Another problem with a tantalum seed layer is that, at elevated temperatures, it can react with the adjacent free layer of a top spin valve and diminish spin valve performance. This becomes more problematic as free layer thickness is reduced.
The present invention provides an improved top spin valve and method of fabrication. In the preferred embodiment of the top spin valve of the present invention, a seed layer is formed of material having the elements Ni and Cr. On the seed layer, a free layer is formed, with a spacer layer overlying the free layer and a pinned layer overlying the free layer to form a top spin valve.
In the preferred embodiments, the seed layer is formed of a material that has an ion milling rate comparable to that of the free layer material. This allows the free layer to be formed by ion milling with sidewalls having shorter tails. Shorter tails improves the junction between the free layer and an adjacent magnetic bias layer to improve domain structure within the free layer.
In one embodiment, the seed layer may have NiFeCr, with Cr from about 20% to 50%. In another embodiment, the seed layer may have NiCr, with about 40%. Some embodiments may have the seed layer formed on an optional Ta pre-seed layer.
In preferred embodiments of the present invention, the seed layer of the present invention provides several advantages over convention seed layers. The seed layer of the present invention provides an improved fcc (111) texture for NiFe and for NiFe/CoFe free layers grown on a seed layer of NiFeCr, on a seed layer of NiFeCr/Ta, on a seed layer of NiCr, or on a seed layer of NiCr/Ta.
Furthermore, the seed layer of the present invention also provides improved texture in other overlying layers, such as a D spacer layer, a pinned layer, and a pinning layer. Improving the texture of the pinning layer improves the exchange bias field resulting in better top spin valve thermal stability.
Improving seed layer texture is particularly important as free layer thickness is reduced and seed texture becomes more critical to providing good texture to the free layer and the overlying layers. As such, the seed layer of the preferred embodiments allows optimization of spin valve performance for spin valves with free layers without NiFe, such as a free layer of CoFe.
In preferred embodiments of the present invention, the seed layer provides high resistivity, which minimizes shunting of sensing current. Furthermore, the NiFeCr seed layer and the NiCr seed layer of the preferred embodiments of the present invention are more thermally stable and less reactive with NiFe free layer than Ta. In addition, in preferred embodiments, the seed layer of the present invention improves the magnetostriction of adjacent NiFe free layer material. Moreover, in some embodiments of the present invention, the seed layer may be utilized improve the soft properties of a CoFe free layer without using NiFe or other adjacent magnetic portions or layers.